bcl-2 and tamoxifen-induced apoptosis

Tamoxifen (TAM) is commonly used for adjuvant therapy of breast cancer, and is believed to work
by competing with estrogen for binding to the estrogen receptor (ER). It has been suggested
that TAM may exert its anti-proliferative effects by inducing apoptosis. The bcl-2 family
of genes are associated with apoptosis, with the gene products bcl-2, bcl-XL being
inhibitors, while others such as bax are inducers of cell death. Apoptosis is controlled by the
ratio of the various bcl-2 family members. Overexpression of HER2 in ER-positive MCF7 cells
has been shown to suppress tamoxifen-induced apoptosis by up-regulating bcl-2 and bcl-
XL protein. However, it is not known whether TAM directly modulates bcl-2, bax or bcl-XL.

Expression of bcl-2 appears to be a favourable prognostic factor and predictor of response to
endocrine therapy. Frequent co-expression with ER may indicate that the pathways are
closely linked in breast epithelial cells, and reflects the importance of sensitivity to estrogens
and apoptosis in the development and progression of breast cancer. The improved response
to hormonal treatment of bcl-2-positive tumors may be explained by TAM-induced
apoptosis via down-regulation of bcl-2 levels in these cells. The lack of association between
tamoxifen and other bcl-2 family members described here may explain the reported lack of
prognostic and predictive reliability of these gene products.

MCF-7 breast cancer cells were exposed to TAM, and apoptosis measured by an
enzyme-linked immunosorbent assay (ELISA). Semi-quantitation of bcl-2, bax and bcl-
XL levels were performed using reverse transcriptase polymerase chain reaction (RT-
PCR). Protein quantitation for bcl-2, bax, bcl-XL and p53 was carried out by western blot
analysis.

TAM-induced apoptosis in MCF-7 cells was seen to be time- and concentration-dependent. TAM decreased the expression of bcl-2 mRNA and protein in a dose dependent manner. Expression of bax and bcl-XL mRNA and protein were unchanged with TAM treatment.The time-dependent down-regulation of bcl-2 protein
correlated with apoptosis. TAM did not affect p53 protein levels.

The data presented here demonstrate that TAM can induce apoptosis in a time- and
concentration-dependent manner by modulating bcl-2 levels in breast cancer cells. This
down-regulation of bcl-2 was not accompanied by alterations in bax, bcl-XL or p53
levels.